[newbie-contributor] New contributor docs review — local upload command uses directory path with --zip

## Summary
The local-development upload instructions currently provide a command that passes a directory path to `elastic-package install --zip`, which expects a package zip file. New contributors following the documented path can fail at the upload step.

## Findings

### 1. Local upload example uses a directory path instead of a zip file

**Where:** `docs/extend/upload-new-integration.md` ("Local development"), line 20
**Problem:** The documented command is:

```bash
elastic-package install --zip /path/to/my/custom-integration
```

This path is a directory, not a zip artifact.

**Evidence:** In the same docs set, `--zip` usage is shown with zip files:
- `docs/extend/elastic-package.md` line 173: `elastic-package install --zip /home/user/Coding/work/integrations/build/packages/elastic_package_registry-0.0.6.zip -v`
- `docs/extend/elastic-package.md` line 207: `elastic-package install --zip elastic_package_registry-0.0.6.zip -v`

**Impact:** A new contributor following the "Upload an integration" local workflow can hit a hard failure at the install/upload step, blocking validation in Kibana without additional troubleshooting.

**Suggested fix:** Update the local-development command to reference a built zip file path (for example from `build/packages/*.zip`) or switch to `elastic-package install` from the package directory if that is the intended path.

## Suggested Actions

- [ ] Replace the local upload example in `docs/extend/upload-new-integration.md` with a valid zip-file path for `--zip`, or use the non-`--zip` command with the expected working directory documented.

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